1. Field of the Invention
This invention relates to a real image mode finder optical system which is suitable for compact cameras such as photographic cameras and video cameras.
2. Description of the Related Art
In general, finder optical systems are available in virtual image modes, such as Albada and inverse Galilean types, and real image modes, such as Keplerjan types. The finder optical systems of conventional compact cameras has found many uses for virtual image modes. It is for this reason that the virtual image mode finder optical system brings about a wide field angle with comparative ease, has a smaller number of parts because it dispenses with an image erecting means, and allows its length along the optical axis to be reduced. Recently, on the other hand, the real image mode finder optical system has undergone many improvements such that an indicating member, like a field frame, is made easy to see, the effective diameters of lenses can be reduced, and an image inverting optical member, like a Porro prism, can be integrally molded of plastic. Consequently, real image mode finder optical systems (such as those set forth, for example, in Japanese Patent Preliminary Publication Nos. Sho 63-44616 and Sho 61-156018) have come into prominent use.
With the conventional real image mode finder optical systems as mentioned above, however, when an attempt is made to secure a relatively wide field angle, the height of an object image formed by an objective system will inevitably increase. Thus, for compactness of the optical system, the refracting power of the objective system need be increased. This causes the degradation of aberration. Hence, in order to improve the performance of the optical system, it is indispensable to increase the number of lenses of the objective system and lessen the refracting power of each lens. As a consequence of this, the problem is raised that the number of parts used in the optical system increases and its arrangement is complicated. Moreover, the position of the object image formed by the objective system is located in front of the image inverting optical member, and an eyepiece is located behind it.
FIG. 1 show the arrangement of the conventional real image mode finder optical system. This optical system includes an objective lens having lens surfaces R.sub.1 and R.sub.2 ; an image inverting optical member having lens surfaces R.sub.3 and R.sub.4 and reflecting surfaces M.sub.1, M.sub.2, M.sub.3, and M.sub.4 ; and an eyepiece having lens surfaces R.sub.5 and R.sub.6. Thus, when the finder optical system is actually disposed in a camera body, the objective lens and the eyepiece, as shown in FIG. 1, are situated in front of and behind the Porto prism of the image inverting optical member, no matter how the arrangement of the optical system is simplified. Hence, the corresponding thickness of the optical system is imperatively needed, and there is a limit to the reduction of the length along the optical axis.
In order to solve such a problem, optical systems, such as those disclosed in Japanese Patent Preliminary Publication Nos. Sho 63-226616 and Hei 1-255825, are available. These optical systems are designed so that the position of an object image formed by an objective lens is located inside an image inverting optical member to thereby intend the reduction of the overall length. Such prior art, however, develops the defect that because an optical member, such as a field frame, is disposed at the imaging position, the image inverting optical member, such as the Porto prism which can be integrally molded originally, must be divided into two, with a resulting increase in the number of parts.